A successful hip replacement or arthroplasty procedure results, in part, from selection of prosthetic joint components that are dimensioned and positioned to closely approximate or replicate the geometry and functional characteristics of a natural, healthy hip joint. Typically, the component selection process includes a pre-operative analysis of joint images. The component selection process also includes temporary fixation of one or more provisional components to a bone or bones of interest prior to permanent fixation of the prosthetic joint. The provisional components are intended to mimic certain aspects of the permanent prosthetic joint in order for a surgeon to validate measurements and to test several different component sizes and configurations. Hence, provisional components are aptly known as “trials.”
Two different types of procedures are typically suitable for hemiarthroplasty procedures. Briefly, hemiarthroplasty is a surgical procedure in which a joint is partially replaced, i.e., the natural acetabulum is retained in the hip joint of the patient. One of these types is the use of a bipolar prosthesis member (for example, see U.S. Pat. No. 3,813,699 to Giliberty and U.S. Pat. No. 4,770,658 to Geremakis et al.). In general, a bipolar prosthesis has an external surface which articulates with the natural acetabulum and an internal surface which articulates with the spherical head member of a prosthetic femoral component. The other of these types is often referred to as a unipolar endoprosthesis in which the prosthetic femoral component includes a spherical head member which is large enough to articulate directly with the natural acetabulum. Both of the above hemiarthroplasty procedures enable articulation with the natural acetabulum.
These two procedures permit later conversion to a total hip replacement in which the acetabular portion is also replaced with a prosthetic acetabular component. With the bipolar procedure, the bipolar prosthesis is removed from the head of the hip stem, and an acetabular prosthesis is implanted which mates with the head of the remaining femoral component. With the unipolar endoprosthesis, the head is typically a separate component from the stem portion of the hip prosthesis. This permits the larger unipolar head to be removed and replaced with a smaller head without removing the femoral stem to enable the smaller head to mate with an acetabular prosthetic implant component
Determining the fit of a prosthesis or medical implant before implantation is generally accomplished using a trial prosthesis instrumentation. An example of a femoral prosthesis trial fitting device is described in U.S. Pat. No. 4,135,517 and corresponding application GB 2001247 to Reale, which disclose a trial head 30 and a bearing insert which removably fits within the trial head. The bearing insert may be removably mountable on either a femoral prosthesis stem or a trial handle.
U.S. Pat. No. 5,879,401 to Besemer et al. describes an acetabular trial system comprising an outer shell and an inner shell for placement within the outer shell that are rotatably fixed with a pin about a transverse axis that extends through the outer shell and that defines a range of anteversion angels, wherein the transverse axis is perpendicular to a sagittal plane of a patient's body. This trial system allows for the determination of an appropriate anteversion angle for the acetabular cup of the hip prosthesis. U.S. Pat. No. 4,528,980 and corresponding application EP 0140642 to Kenna describe an acetabulum sizer for properly sizing an acetabulum prior to receiving an acetabular cup prosthesis comprising a substantially hemispherical shell having an outside surface that generally conforms to the outside surface of the prosthesis, a circular peripheral rim, a handle means for manipulating the shell, wherein the shell is characterized by having viewing ports to enable visual inspection of the acetabulum during sizing.
In an effort to simplify the trial process of a surgical procedure and expand the utility of conventional trial prosthesis instrumentation, combined bipolar and unipolar trial instrumentation were explored. For example, related U.S. Pat. No. 5,800,556 and U.S. Pat. No. 5,888,211 to Sanders et al. describe a bipolar to unipolar head trial adapter that may be inserted into or removed from a bipolar shell to convert the bipolar shell to a unipolar head. The adapter comprises a two piece system that provides an adjustable neck to eliminate the necessity for multiple unipolar head trials corresponding to different neck lengths for each of the different possible head sizes. U.S. Pat. No. 5,156,626 describes a multiple piece set of provisional instrumentation for a hemiarthroplasty that includes a common outer shell that releasably mates with two different types of inner components, wherein the combination of the shell and the first inner component is used for a bipolar procedure and the combination of the shell and the second inner component is used for a unipolar procedure.
The present invention further simplifies the trial instrumentation employed in trial fitting of a medical implant (i.e., joint prosthesis) by providing a two-piece trial prosthesis instrumentation for use in either a bipolar or a unipolar surgical procedure.